[meteorite-list] fire flies or flying fires

[Chris Peterson]

Hi Sterling-

Yeah, I guess you're right, we're doomed to disagree <g>. (Did you see the
cleverly packaged MIT T-shirts distributed to MIT freshman at last years
orientation? The ones that nobody noticed until too late had "Because not
everyone can go to Caltech" printed on the back?)

I take exception to your point that the Moon "obviously" has a uniform crust
early on. This isn't obvious at all, and nobody has a good explanation for
why the Moon does not now have a uniform crust. This feature does not fall
out of any models. Gravity does not obviously explain why the crust should
be thinner towards the Earth. If the crust thickness variation developed
early (and the maria are certainly old) this would explain the reason that
maria are only present on one side. Since it is likely that the Moon was not
yet tidally locked when the basins formed, I don't see the effects of the
Earth as having contributed in an obvious way to their formation.

Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


----- Original Message ----- 
From: "Sterling K. Webb" <kelly at bhil.com>
To: "Chris Peterson" <clp at alumni.caltech.edu>; 
<Meteorite-list at meteoritecentral.com>; "Dawn & Gerald Flaherty" 
<grf2 at verizon.net>
Sent: Monday, June 27, 2005 3:55 AM
Subject: Re: [meteorite-list] fire flies or flying fires


> Hi,
>
>    There are a variety of questions involved.
>
>    First, there is to the problem of identifying what constitutes a 
> "basin" as
> the earliest basins are relicts of the earliest impacts and are obscured 
> to a
> greater or lesser degree by later large basin forming impacts.  Most
> selenologists think Procellarum is the most ancient relict basin overlaid 
> by the
> later Imbrium, Serenitatis, Tranquilitatis, Humorium and Nubium basins.
>
>    If Procellarum is a basin, it would be the largest on the Moon at 3200 
> km
> (near side), followed by South Polar basin at 2500 km (on the edge  but 
> mostly
> near side), followed by Imbrium at 1160 km (near side), Crisium at 1060 km 
> (near
> side), Orientale at 930 km (on the edge, side), down through a long list 
> sorted
> by size, until you arrive at the Keeler-Heavyside basin at 780 km (far 
> side).
>
>    The size of the basin is not linearly related to the size of the 
> impactor,
> as the state of the Moon was changing throughout the period of 
> bombardment.  The
> crust, its distribution and other characteristics, the mantle, the core, 
> were
> continually evolving over the basin-impact period.
>
>    Obviously, when the Moon was mostly molten and starting to 
> differentiate,
> the crust was a new thin skin everywhere.  Procellarum, if a basin, formed
> within the Moon's first 150 million years, before (during?) mantle
> crystallization and would not have required as large an impactor as later 
> basins
> of lesser size.  At this point the differential crust thickness and the
> eccentric mass center lay (mostly?) in the Moon's future.
>
>    The gravitational effects that displaced the Moon's center of mass 
> toward
> the Earth only take effect after cooling and differentiation has 
> progressed to a
> sufficiently great degree and the crust differential only develops late or 
> after
> that mass center shift is mostly complete, then it "freezes" in place.
>
>    Imbrium, the second largest basin, formed when the Moon was 600 million
> years old (?), is thought by many to have been the target of the largest 
> impact
> object of the Moon's history, well after the crust differential had 
> developed.
> Indeed, Imbrium appears to be one of the youngest basins in the solar 
> system,
> hence requiring a stupendous whack from an object large enough to be a
> respectable little moon itself.
>
>    Another young (or later, whichever you prefer) giant basin is Caloris 
> on
> Mercury, larger at 3700 km, whose impactor came close to breaking the 
> planet, as
> the vast stretch of chaotic terrain on the opposition point on Mercury's 
> surface
> shows.  Planet breakers, not a happy concept.
>
>    However, the point of all this is that ancient basins are identifiable 
> even
> when overlaid by one or several or many later basins (like Procellarum 
> is).  The
> far side shows no traces, however faint, of former gigantic relict basins 
> to
> rival those of the near side or the poles.
>
>    There was far less magna upwelling on the far side to obliterate traces 
> of
> relict basins.  The thick crust protects relict basins from obliteration 
> because
> it is harder to form new large basins on (or is the word "in"?).  Yet, the
> central far side is the most depleted in traces of ancient basins, 
> implying far
> fewer impacts. Large ringed craters / small basins are pretty much the 
> same
> thing and in uniform distribution.  The Moon has a huge number of them and 
> every
> smaller size of impact, but giant ancient basins?  Near side is the spot 
> where
> the action was.
>
>    We also have to distinguish between ringed basins formed in an already
> stiffened crust and flood basins formed by large upwellings alone, 
> although
> Procellarum is so old it's hard to tell, it doesn't appear to be a ringed 
> basin,
> although it did form on older crust.
>
>    The WHOLE dating question is still problematic.  The oldest fragment
> recovered by Apollo were inclusions formed at 4450 mya (million years ago) 
> which
> would set the Earth-Moon impact (the current theory) at an Earth age of 
> almost
> nothing! But a single crystallized grain is thin evidence.  The next 
> oldest
> samples are mare basalt fragments in younger lunar breccias at 4230 mya,
> implying a lunar magma had upwelled to cool into new crust to be 
> fragmented by
> impact, but that's a big gap.  No complete lunar rock as a whole older 
> than 4150
> mya.  We think.  No samples at all from the far side, of course. Get me 
> 10,000
> more samples and we'll straighten this whole thing out!  Or would we?
>
>    The famous ancient zircon from Australia, now living in the lap of 
> luxury in
> Wisconsin, is 4404 (+/-4) mya, formed in a time of temperate conditions 
> and
> liquid water, they say.  But wait, Time Zero is about 4560 mya, the 
> earliest
> planetesimals start forming at 4555 mya.  The Earth and other terrestrial 
> bodies
> are essential complete by 4500 mya.
>
>    But the Earth-Moon impact theory, with it's 2-3 Mars mass body 
> completely
> remelting an already formed and much smaller complete Earth and adding a 
> whole
> new crust on top of the old crust, with full differentiation taking place 
> AGAIN,
> including a new major addition to the core, when did that happen?  After 
> 4500
> mya (complete first Earth) but before 4450 mya (lunar crystal), right?  Or 
> at
> 4500 mya, at the very last minute?  Why is the happy zircon so happy?  And 
> when
> it find a time and place to be happy in at 4404 mya?
>
>    If the 4450 mya lunar crystal is a lucky inclusion (like the zircon?) 
> was
> there a happy Earth that got whacked by another planetoid and the lucky 
> zircon
> is a chance survivor?  The pro-zircon forces are in favor of a totally 
> Cool
> Early Earth (I think it's pretty cool, myself).  The Earth-Moon Impact 
> crowd
> insist that their Early Earth be served Molten Hot Twice.  Am I the only 
> one to
> see a problem here?  NO Hadean Epoch?  TWO Hadean Epochs?
>
>    The happy zircon is also a funny zircon.  Even it's a tiny crystal, 
> other
> probe spots on it are younger than the 4404 mya spot, even though it's one
> crystal.  Contacting zircons in the one little rock the crystal is 
> included in
> are as young as 3300 mya.  There's no "this one whole microscopic crystal 
> is
> 4404 mya" object.  It has had a complicated life for a happy zircon.
>
>    It's clear the happy zircon is a triumph of micro-probe dating, but 
> what is
> it proof of?  And the Moon?  We know how long it would take the lunar 
> mantle to
> crystallize and we know how long again before the core solidified, but 
> WHEN does
> that timeline start?  The Earth-Moon Impact crowd say the Moon was 
> complete by
> 4440 mya but about how long it took to assemble, there is silence and a 
> few
> mutters, "a couple of days" say some, "100 million years" say some. The 
> models
> are dynamically incomplete.  So sorry.  Recent remodeling says it was 
> "more
> likely" quick, but then we want it, need it, to be quick.
>
>    We are still quarreling about the Late Collisional Bombardment, whether
> there was one or not, for heaven's sake.
>
>    Stitching together the happy zircon timeline, the Earth-Moon impact
> timeline, the planetesimal and accretion timeline, and all the other 
> timelines
> is not complete.  We're still looking for the needle and thread.  Oddities
> abound.  Every living thing on Earth has the same little snippet of 16S
> ribosomal RNA gene (in various related mutated versions).  What is it good 
> for?
> Why, metabolizing sulfur for energy!  We all get our "go" from sulfur, 
> right?
> No.  The critters that need it are the archaic sulfur eating bacteria in 
> deep
> sea hot mineral vents, and some biologists suggest that its universality 
> arises
> from an early extinction of ALL life but them, and that life had to start 
> all
> over again. Earth-Moon impact, anyone? After the happy zircon?
>
>    This stitching is like making a crazy quilt.  Drives me crazy, anyway.
>
>    I'm a lot more certain about the predominance of near side ancient 
> basins,
> though.
>
>    We're doomed to disagree, Chris, you (CalTech) and me (MIT).